kauth: Don't overflow stack when building username

knfs constructs the userName by combining the clientName.name
and clientName.instance arrays, along with a dot separator. Make
sure that the userName array is big enough to hold these, and
use strlcpy and strlcat just to make sure.

Caught by coverity (#985829)

Change-Id: I75431212c8464861a26546c9e47d13acbff08967
Reviewed-on: http://gerrit.openafs.org/9351
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>